Designers of propellant gas pressure operation systems for semi-automatic firearms have found it particularly challenging to produce a system that will operate reliably using ammunition with significant differences in muzzle velocities and chamber pressures. For example, the .300 Blackout (.300BLK) caliber is available in both supersonic and subsonic velocities. Because it uses the same magazine and bolt in an AR15-pattern firearm as those used for .223/5.56 caliber ammunition, the popularity of .300 Blackout has grown in recent years. Users may choose ammunition in this caliber loaded for supersonic velocities to extend its effective range, but may also select subsonic loads to minimize noise by eliminating the transonic “crack” created by supersonic velocities, particularly when used in conjunction with a firearm noise suppressor or “silencer.” As used herein, ammunition “load” means a particular combination of propellant and projectile, for a given caliber, that produces different ballistic results, such as chamber pressure and velocity.
While the exact speed of sound in air varies depending on atmospheric conditions, ammunition with a muzzle velocity less than about 1,040 feet per second (FPS) is generally considered “subsonic” in that it does not exceed the speed of sound and the transonic “crack” associated with it. For example, subsonic .300BLK ammunition using a 220 grain bullet can have a chamber pressure of approximately 21,000 pounds per square inch (PSI) and a muzzle velocity of approximately 1,010 FPS. In contrast, a .300 Blackout cartridge loaded for supersonic velocities may use a 125 grain bullet to produce a chamber pressure of approximately 55,000 PSI and a muzzle velocity of approximately 2,215 FPS. Because of these vastly different specifications, it has been difficult to design a gas-operated semi-automatic system that will operate reliably using both types of ammunition, even though it is the same caliber. This is particularly the case with direct impingement operating systems, the most common for the AR15 platform.
Prior solutions have focused on adjusting the flow of operating gas delivered to the bolt carrier assembly of a direct impingement system. For example, systems have selected between one or more than one of multiple barrel ports located at or near the same longitudinal position. This adjusts gas flow by increasing or decreasing the total cross-sectional area of the gas passageway. But adjusting gas flow alone is inadequate to provide reliable operation for ammunition producing vastly different projectile velocities and pressures. Typically, such systems operate reliably with either supersonic or subsonic ammunition, but not both. Systems that seek to provide a compromise between the demands of these vastly different ammunition loads ultimately may not provide reliability for either.